Leveraging the potential of low power so-called green or renewable power sources such as photovoltaic (solar cells), fuel cells, wind and other such power generation technologies requires efficient and well-behaved converters. The power sources have a DC power output with relatively low power density and low level voltage. Many loads require higher voltage AC power. Inverting converter architectures are typically used to perform power conversion from such power generation technologies. Other loads can require higher voltage DC power, which also requires an up conversion from a renewable power source.
An initial DC/DC stage is most often used to initially boost voltage levels in a conversion for use with an AC load. A common architecture uses DC/DC up conversion stage follow by a DC/AC converter that is isolated from a load by an often bulky transformer. The bulky 60-Hz line transformer is not acceptable for compact applications. Another common architecture combines a DC/DC converter with a high-frequency transformer followed by a DC/AC converter, which is connected to a load/grid. A third common architecture is a variation of the last mentioned, but includes the high-frequency transformer in the DC/AC conversion stage. These latter two arrangements are known as high-frequency link (HFL) converters. In addition to the high-frequency transformer location, the location of an intermediate fixed, variable, or pulsating dc-link bus (often using an electrolytic capacitor) differs in these architectures.
These HFL converters are suitable for compact power electronics packaging and are, therefore, suitable for power-dense inverter applications. However, the HFL converters have limitations. Generally, the number of devices and the complexity of the drivers and protection circuitry are high in such HFL multi-stage converters. Further, and as mentioned earlier, some of the high-frequency converters may include intermediate stage high-voltage DC-link electrolytic capacitors that are bulky and have long-term reliability issues. Control is difficult in these arrangements because each of these multi-stage converters have to be controlled, which raises control complexity and computational burden. The arrangements are not always readily scalable because topology is often functionality dependent and a generic architecture may come at the price of higher number of devices and associated drive and protection complexities. These architectures also have more difficult path to achieving desirable efficiency since loss of each stage of the multistage converter has to be controlled via careful and complex design.
A single-stage converter, which alleviates some of the above-mentioned complexities and challenges in multi-stage HFL converters, would provide system-level value added from the simultaneous standpoints of cost, power density, reliability, and efficiency.
One isolated HFL single-stage differential inverter (i.e. DC-AC converter) was disclosed in S. K. Mazumder, R. K. Burra, R. Huang, and V. Arguelles, “A Low-Cost Single-Stage Isolated Differential Cuk Inverter for Fuel Cell Application”, IEEE Power Electronics Specialists Conference, pp. 4426-4431 (2008) for fuel cell applications based on the Ćuk topology. Ćuk converter for DC-DC applications was developed originally by Slobodan Ćuk and has been extensively researched. See, e.g., S. Ćuk and Z. Zhang, “A High Efficiency 500 W Step-up Ćuk Converter,” Proc. of Power Electronics and Motion Control Conference (2000); D. Maskimovic and S Ćuk, “Switching Converters with Wide DC Conversion Range”, IEEE Transactions on Power Electronics, 1991.
However, as mentioned above, the 2008 disclosure by S. K. Mazumder et al. was for an inverter (DC-AC) and not for a DC-DC application and had a differential mode. While this concept provided a low-circuit-count design, problems with this design related to scalability, noise components, and nonlinear voltage gain, and closed-loop control remained unresolved. Further, the signal-phase architecture in this paper would require six modules for providing three-phase power output. This increases device count and circuit complexity, while reducing efficiency and reliability. Inductors and transformers in the circuit in this paper were on a common core, but the circuit retains DC components of the dynamics. The control was via a simple PI (proportional integral controller) that had only a voltage loop. The nonlinearity of the voltage gain of the converter could not be adequately handled with this control, and that control was only demonstrated for the particular circuit and inverter type.